A proton exchange membrane (PEM) is a semi-permeable membrane typically made from ionomers designed to conduct protons while being essentially impermeable to gases such as oxygen or hydrogen. Thus, separation of reactants and the transport of protons is the basic function of a PEM when used in a proton exchange membrane fuel cell.
A PEM can be made from either a polymeric material or a polymeric composite membrane where other materials are embedded into the polymer matrix. Nafion® is one of the most common and commercially available PEM materials on the market today. Nafion® is a sulfonated tetrafluoroethylene copolymer which incorporates perfluorovinyl ether side chains terminated with sulfonic acid groups. However, Nafion® has limitations such as an upper operational temperature limit of about 80° C., a high permeability to methanol, the release of fluorine upon its decomposition and high cost. Due in part to these limitations alternative membrane materials have been, and are currently being, researched for suitable alternatives and replacements.
Alternative polymers to Nafion® that have been researched include polybenzimidazoles, poly(phenylene oxides) and poly(arylene ethers), all of which contain aryl rings in the polymer backbone. Polymers such as these do not inherently conduct protons and must be modified to incorporate acidic functionalities, usually sulfonic acid groups. The modification is typically accomplished by sulfonation of the polymers with SO3, concentrated H2SO4 or ClSO3H. Unfortunately, these sulfonated polyarylenes suffer from numerous problems including lower ionic conductivity or poor dimensional stability in water at high ion conductivity and form low oxidative stability Due to low oxidative stability, questions remain about the lifetime of membrane-electrode assemblies containing these sulfonated polyarylenes.
Polyphosphazenes have also been considered for PEM materials. Polyphosphazenes are polymers that possess a backbone of alternating phosphorus and nitrogen atoms, wherein each phosphorus atom is linked to two organic, inorganic, or organometallic side groups. Factors affecting the design of a successful PEM material may include thermal, mechanical, and chemical stability; barrier properties; and water uptake/hydrophobicity. Currently, polyphosphazenes that have yielded the best combination of properties have been aryloxy substituted materials. For example, Pintauro et al. in U.S. Pat. No. 6,365,294 disclosed sulfonated polyaryloxy substituted phosphazenes such as poly[bis(3-metlhylphenoxy)phosphazene], poly[(3-metlhylphenoxy)(phenoxy)phosphazene], poly[(3-ethylphenoxy)(phenoxy)phosphazene], poly[3-methylphenoxy)(3-ethylphenoxy)phosphazene], and the like. In addition, Hiroshi Akita has disclosed in U.S. Patent Application Publication No. 2005/0014927 a polyphosphazene derivative and an aromatic ring compound bonded to one another to obtain an intermediate product. In particular, Hiroshi Akita has disclosed a sulfonated polyphosphazene derivative wherein a sulfonic acid group is bonded to an aromatic ring and the average molecular weight of said derivative is not less than 25,000.
Current sulfonated phosphazene polymers are hindered by excess water swelling when said polymers approach the ion exchange capacity and resulting high ionic conductivity levels required by proton exchange membrane fuel cells. For example, solution-cast membranes made from sulfonated polymers with proton conductivities greater than 0.45 S/cm have exhibited very poor dimensional stability in water (Rozière et al. Annu. Rev. Mater. Res. 2003. 33, 503-55, FIG. 14).